Networks for data communication may be equipped with passive and/or active star couplers between communication channels,
Passive star couplers are used in optical communication networks. A passive star coupler is defined as having no amplifying or regenerating components therefore needing no electrical energy. The power of the optical flow of data is split by optical means. A star coupler is provided with ports, each port is provided with an input and an output. The star coupler is provided with as many inputs as outputs.
Each single station of the network is connected by two light waveguides labeled LWL in the following, to the star coupler. One LWL is transporting data to be received while the other LWL is transporting data to be transmitted by the station. A pair of these LWLs is associated to a port of the star coupler. The number of ports of the star coupler and the number of stations of the network are the same. The signals having power P and supplied to the input of a port by a LWL are split into nearly equal amounts by the order of magnitude of P/n (without considering internal coupling Losses) to the outputs of the star coupler. From the outputs the optical signals are transmitted on one of the n LWL to a receiver installed in one of the n stations of the network.
The output of that port whose input receives incoming signals is provided by the same amount of power as the outputs of all other ports, Star couplers of this type are designated "symmetrical" in the following if
1. The number of inputs is equal to the number of outputs, and
2. the optical power supplied to the input of any port X is split nearly equally among the outputs of all ports i.e. to the output of the port X too.
It is essential that the maximum working range of transmission i.e. the maximum distance between two stations, Lmax, depends on the number of ports of the star coupler, i.e. the number of stations of the network.
For optical data networks symmetrical active star couplers may be used too, the definition of symmetry mentioned above is equally valid. Such a star coupler is provided with n ports, each port being provided by one input (with an optical receiver) and one output (with an optical transmitter). Therefore the star coupler is provided with as many inputs as outputs. A star coupler as mentioned above is defined in accordance with the corresponding passive star coupler as a symmetrical active star coupler.
The star coupler as mentioned above is symmetrical because it is provided by as many inputs as outputs and because each output including the output of that port whose input receives the optical signals is fed by the same amount of signal power. The star coupler is active because all received optical signals are converted to electrical signals, regenerated with respect to Level and timing, fed to all outputs by an electronic circuit and retransferred at the outputs in optical signals and because electrical energy is required for these steps.
It is essential that the maximum working range of the network i.e. the maximum distance between two stations, Lmax, is independent of the number of ports of the star connector, i.e. the number of stations, if an active star coupler is used.
Networks may be provided with active and passive star couplers. Passive star couplers are safe against failure to a high degree.
However passive star couplers are suitable for networks with a limited range and number of stations only. Active star couplers are suitable for networks of very large working range and many stations.
Active star couplers are equipped with many optoelectronic converters and electronic circuits rendering them more expensive as compared to passive couplers in general. Moreover there are additional provisions necessary to increase safety against failure. On account of these disadvantages it is not reasonable for large, extended area networks having many stations and large ranges of transmission to use one star coupler exclusively though this might be possible in principle: A single active star coupler to which all stations of a network are connected will require Large precautionary measures to increase safety as well as large expenses for cabling: Several stations which are located close to each other e,g. forming clusters, would have to be connected to the distantly located active coupler by individual light waveguides. It is more convenient for large networks to provide several star couplers not only active but also passive ones to include the advantages of passive star couplers too:
Several stations are connected in groups of typically 8 possibly more, e.g. 16 or 32 stations by a passive star coupler. Several of these subnetworks having passive star couplers are connected to each other by an active star coupler.
Several networks of this type provided with active star couplers are connected themselves in groups of e.g. 8 networks by passive star couplers and so on. In this way one gets a network of cascaded active and passive star couplers. This network may conveniently be adjusted to increasing data rates and increasing stations e.g. of an industrial automation system.
For such networks having cascaded active and passive star couplers there must be solved an important problem: The message transmitted by a station must be prevented from arriving at any one of the receivers of the linked stations on several, different channels e.g. several times successively.
This will happen inevitably, if both the symmetrical passive and the symmetrical active star coupler as well are used commonly for setting up networks.
A network provided with symmetrical active and symmetrical passive star couplers used at the same time is therefore not functional.
This problem may be solved by networks provided with asymmetrical active and passive star couplers and a single symmetrical active or passive star coupler. This is accomplished by splitters and combiners which are specially designed types of the general star couplers.
A splitter is provided with n inputs and n outputs and splits the light supplied to its input equally to its n outputs. A combiner is provided with n inputs and one output and transmits each light signal supplied to one of its inputs to the common output.
Splitters and combiners are asymmetrical star couplers. They may be implemented as passive or active units. Many vendors are offering passive units. The ratio of power PE supplied to an input to the distributed power
is for splitters and combiners in the order of magnitude as follows: PE/PA=n, in which n means the number of outputs of the splitters or combiners and internal losses are neglected. From this follows the dependance being principally the same for splitters or combiners with regard to maximal number of stations and range of a network if symmetrical passive star couplers are used.
The operating characteristic of active splitters and combiners as well as of active symmetrical star couplers consists in converting the optical signal received by an optical receiver into an electrical signal which is distributed electrically to an output or to the outputs and possibly regenerated before converting it into an optical signal in an optical transmitter at the output. Maximum range and number of stations of network provided with active splitters or combiners are not correlated.
With regard to advantages and disadvantages of active and passive splitters and combiners the above mentioned statements made for symmetrical star couplers are valid as well. In this context the problems concerning the safety against failure of active splitters and combiners must be mentioned. A splitter provided with n outputs and a combiner provided with n inputs may be combined to a so-called "splitter/combiner".
A splitter/combiner is a specially designed star coupler. It is provided with as many inputs as outputs, however it is not a symmetrical star coupler according to the above made definition because the light supplied to any one of its inputs is not split equally to all provided outputs: The light supplied to input n+1 will be distributed equally to outputs 1 to n however output n+1 will receive no light. If light is supplied to one of the inputs 1 to n, light is transmitted to output n+1 only while outputs 1 to n receive no light. Splitter/combiners are used as passive as well as active units.
Networks splitters/combiners are used to combine groups of stations or sub-networks (Parts of networks). Combining stations or sub-networks group by group according to this principle will result in an uppermost point of a hierarchical network generated by this procedure. At this uppermost point a symmetrical star coupler either passive or active will be installed depending on the star coupler (splitter/combiner) type in the next Lower level being active or passive.
Splitters/combiners are used on all levels of a hierarchical network to combine or split flow of data. In the respective levels of the hierarchy passive and active splitters/combiners are used alternatively. All data flow transmitted from below e.g. from a lower level is inevitably transmitted to a level above and never to other stations or sub-networks at the same level or a lower level directly. Vice versa flow of data received from above is inevitably transmitted below and never above. This is defined as "up-stream" or "down-stream" flow of data.
The real "turn table" for reversing the flow of data from up-stream to downstream is the uppermost point of the network, where a symmetrical star coupler is functioning as "loop-back-point".
This coupler distributes according to the above mentioned definition all signals supplied to an input to all outputs equally, to the output of that port too whose input received the signals. For the network it is essential that there exists a single symmetrical star coupler. The above described topological structure prevents circular signal currents because a receiver is supplied by a transmitted message only once.
A message which is to be transmitted to an immediately neighbouring station in the same cluster will not arrive at this station on the shortest link but has to pass up-stream all levels of the hierarchical network and will be reversed at the uppermost point before passing all levels of the hierarchical network down-stream to arrive at the neighbouring station of destination finally. Assuming a small distance between this station and the transmitting station and a distance covered through the light waveguide of approximately 3 kilometers and assuming further the message to pass up-stream and down-stream must pass through additional active splitters/combiners prone to failure or the active star connector prone to failure at the top of the network (Loop-back-point), then there are many possibilities for hazards and distortions of the data communication which is an intolerable situation with regard to the direct vicinity of both stations communicating to each other. Communication between stations connected together functionally is interrupted if only one of many superior levels of network or one of the components connecting the levels, e.g. a component located far away at the loop-back-point of the network or in its neighborhood breaks down.
A second disadvantage is economical: Local area networks should be flexible e.g. they should be able to grow with increasing number of stations.
If a network is to be equipped with 8 stations e.g. in the beginning these 8 stations will be connected to a symmetrical passive star connector. If the number of stations is to be increased to 16 e.g. all 8 stations may be connected to a splitter/combiner providing two subnetworks which may be connected to an active star coupler. Therefore the passive symmetrical star coupler bought and used in the previous network is useless.
It is an object of the present invention to improve a star-shaped network for data transmission between stations being provided with symmetrical passive star couplers and asymmetrical active star couplers so that messages transmitted from any one of the stations are prevented from arriving at the same receiver on several, different channels (ways) successively.